A TN (twisted nematic) mode LCD is a common type of conventional LCD using liquid crystal molecules that have positive dielectric anisotropy and are horizontally aligned in a twisted state between two facing substrates. However, the TN mode LCDs cannot display an absolute black state because of light leakage attributable to the birefringence due to the liquid crystal molecules near the substrates even when in an OFF-state. Meanwhile, there are LCDs to which various modes have recently been introduced so as to increase the width of viewing angles. Out of them, IPS-LCDs can display an almost complete black state in an OFF-state by way of the alignment of polarizers on the upper and lower surfaces of the substrates because the liquid crystal molecules are almost horizontally and uniformly aligned to the surfaces of the substrates in an OFF-state so that the light undergoes no change in the polarizer and thus can pass through the liquid crystal layer intact.
Without optical films, generally, wide viewing angles can be realized in these IPS-LCDs which thus assure high transmissivity and have uniform images and viewing angles over the screen. For these reasons, IPS-LCDs are prevalent in high quality displays having 18-inch or larger screens. In contrast, VA (vertically aligned) mode LCDs impart a display with a fast response speed, but have low transmissivity due to the presence of retardation films between liquid crystal cells and polarizers, required to solve the problems of phase difference and light leakage. Particularly, when subjected to external pressure, VA mode LCDs suffers from the disadvantage of low uniformity and stability due to liquid crystal dynamics.
Various examples of VA mode LCDs to which retardation films are applied to solve the problems of phase difference and light leakage are found in Japanese Pat. Laid-Open Publication Nos. 2003-262869, 2003-262870, and 2003-262871, in which a first and a second retardation plate, each consisting of at least one uni- or biaxial retardation film, is disposed between a first polarizer and a liquid crystal cell and between a second polarizer and the liquid crystal cell, respectively. The retardation film for use in compensating for viewing angles comprises an A-plate for compensating for in-plane retardation (Rin) and a C-plate for compensating for retardation in the thickness direction (Rth) that are placed properly so as to compensate viewing angles.
Examples of the IPS-LCDs which use optical retardation films to improve viewing angles are described in Korean Pat. Laid-Open Publication Nos. 2005-0031940, 2003-0079705 and 2005-0039587, the last invented by the present inventors, where a +A-plate and a +C-plate are arranged between the liquid crystal layer and the polarizer.
In the art, much effort has continued to be made to widen viewing angles by way of, for example, novel and versatile lamination structures and configuring of retardation values.